1. Field of the Invention
The invention relates to a circuit arrangement for controlling a large number of printing electrodes for the non-mechanical parallel printing of character elements or image elements, the printing electrodes being combined to form groups, each of which has a common switching point, corresponding printing electrodes of each group being associated with a common voltage amplifier via electronic control elements, a selected printing electrode being controlled by the switching of a group switch and an amplifier.
2. Description of the Prior Art
For printing and facsimile systems, use is made of devices which utilize electrical digital or analog character signals or image signals in order to compose the corresponding image from separate image elements on a record carrier, for example, paper. The image forming electrodes (printing electrodes) are arranged in a fixed printing electrode array so that one printing electrode is associated with each image element (line parallel printing). A matrix arrangement of the printing electrodes is also known.
U.S. Pat. No. 2,955,894 discloses an electrostatic printing method where the printing electrodes are arranged in a row or a matrix. According to this printing method, an image element is produced in that a gas discharge which produces the charge carriers for the electrostatic charging of the image element is ignited when a given voltage threshold value between the printing electrode and the record carrier is exceeded.
Because a very large number of printing electrodes is required in accordance with the desired resolution of the image, coincidence methods are used for the selective control of these printing electrodes. The number of electronic switches or amplifiers and hence the total number of electronic components can be substantially reduced by means of these coincidence methods.
For example, from German Pat. No. 19 46 815 it is known to combine the printing electrodes to form groups which can be selectively and sequentially controlled. Mutually insulated counter-electrodes are then provided on the rear of the record carrier. A group of printing electrodes is selected by supplying the counter-electrode situated opposite this group with a voltage pulse whose amplitude is proportioned so that the threshold voltage for the ignition of a gas discharge between record carrier and printing electrodes is exceeded. The standard ignition voltage is then the sum of the supplied printing electrode voltage and the supplied counter-electrode voltage. Because printing electrode voltages are always lower than the ignition voltage, a gas discharge required for printing can be produced only in coincidence with a voltage pulse on a counter-electrode. Corresponding printing electrodes of the individual groups are interconnected, so that the number of printing electrode switches equals the number of printing electrodes of a group. The total number of printing electrode switches and counter-electrode switches is reduced to 2.multidot..sqroot.Z in the most favourable case, Z being equal to the total number of printing electrodes.
According to this coincidence method coincidence exists between the discrete electrical image signals present on the printing electrodes and the selection pulse of a counter-electrode for the total printing duration of the selected printing electrode. The separate printing electrode groups successively participate in the printing operation.
German Auslegeschrift No. 18 00 137 discloses the control of the printing electrodes of a thermographic printer in which each resistance element (printing electrode) is connected to a thyristor. Using group switches, a separate group of resistance elements can be selected and a current which suffices for printing can be produced through these resistance elements. The required switching time is small with respect to the printing time required for an image element, so that all groups can be switched on within a short period of time after which all resistance elements simultaneously participate in the printing process. For the same voltage decrease the same current flows through all resistance elements participating in the printing process. A voltage which is variable for the individual resistance element, and hence a variable current, cannot be switched by means of the described circuit arrangement.
The electrostatic and thermographic printing methods do not enable true half-tone printing, because the individual image elements cannot be deliberately varied as regards charge density or size. Only so-called pseudo-half-tones can be realized by element density modulation.
In order to satisfy the more severe requirement as regards the quality of the printed images such as, for example, for the printing of true half-tone images or color images with real color tones, other image printing methods must be used. For example, the electrophoretic printing method known from German Offenlegungsschrift No. 28 08 446 and the Corona method known from German Offenlegungsschrift No. 19 34 890 are perfectly suitable in this respect.
Like the electrostatic printing method, these printing methods involve a large number of printing electrodes for the formation of the separate image elements, so that a device is required for coincidence control of the printing electrodes.
However, because the charge density and the half-tone value of an image element are a function of the voltage on the printing electrode in the latter two printing methods, analog high voltage signals must be processed instead of discrete voltages. A threshold voltage value below which printing no longer takes place, as used for the electrostatic method, does not exist. The processing of analog high voltage signals, however, requires expensive amplifier elements, so that the use of such methods is only feasible if the number of such amplifier elements can indeed be substantially reduced.
Furthermore, contrary to the conventional electrostatic method where the printing time per image element is only some tens of microseconds, printing times of a few milliseconds would be required for a true half-tone reproduction, i.e. true to the original. A coincidence device which successively activates each electrode group would lead to an unacceptable printing time for the customary resolutions of the picture of from 4 to 8 elements per millimeter for a full DIN A4 page. Therefore, if the printing speeds per page which can be achieved by means of the conventional electrostatic printing methods are also to be realized for printing methods such as the electrophoretic method or the corona method which are slow in principle, the only solution will be simultaneous operation of all printing electrodes (parallel printing).